A new method based on optimal voltage sag monitoring program is proposed to locate any type of fault on the transmission line when only voltage sag data are available. Firstly a 0-1 linear programming-based model is built to solve the problem of optimization allocation. The object of the optimization model is to minimize the number of the monitoring buses and the key constraint is to guarantee the observability of the whole system considering any type of fault（balanced or unbalanced）. Then based on the optimal monitoring program， the fault position and faulted transmission line are estimated by implementing the least squares method. In this estimation， only the voltage sag magnitudes recorded at the limited optimal monitoring buses are available measurements. The performance of the proposed method is illustrated in an IEEE 39-bus test system， and the validity of the method is testified.